Patients and providers who say insurers low-balled reimbursement paid under employee benefit plans may be looking for new arguments in this often-thorny legal area.

A federal court earlier this month decided that a group of out-of-network patients and providers could not form a class to sue Aetna for alleged underpayments of their claims. The decision was the most recent setback in a series of cases on behalf of plaintiffs suing over the Ingenix database, which allegedly skewed data to calculate reimbursements below what providers believed were the usual and customary rate for similar services. Groups of plaintiffs sued insurance companies...